(06-10) 14:56 PDT ST. HELENA -- Nearly 400 lightning strikes were recorded early Monday in the North Bay, the National Weather Service said.

"It was quite a lot," said Mark Strudley, a hydrologist with the service.

The unsettled weather was the product of a low-pressure system that dragged moisture up from the south, forecasters said.

The moisture mixed with drier air 15,000 feet over the North Bay, creating an electric charge that released hundreds of bolts of lightning, said Warren Blier, a science officer with the National Weather Service.

"Moisture and instability is being brought in at the mid-levels of the atmosphere," Blier said. "As (the low pressure) takes that moist air and lifts it up, lightning starts developing."

Lightning was observed from St. Helena to Fort Bragg, the weather service said.

Only trace amounts of rain fell across the region.

"These thunderstorms are producing rain," Blier said. "They're just so high up and the air is so dry in between that most of the rain evaporates before it reaches the ground."

And that's bad news, because dry lightning is notorious for starting wildfires.

Twenty small wildfires were reported in the North Bay on Monday, said Suzanne Blankenship, a spokeswoman with the California Department of Forestry and Fire Protection. Thirteen were fully contained by 2:30 p.m., Blankenship said. The largest fire was two acres.

Blankenship said it was unclear how many of the fires were started by lightning.

"Lightning was prevalent throughout the area over the past 24 hours," she said. "However, (firefighters) continue to investigate each fire to rule all other possible causes."